The invention relates to catalysts in tablet form.
Solid catalysts can be used in industrial processes in the form of powders, granules, chips or shaped bodies. Three main processes are employed here for shaping: tableting, extrusion and granulation. These processes are reviewed in Catalyst Handbook, 2nd edition, edited by Martyn V. Twigg, Wolfe Publishing Ltd., 1989, pages 44 to 48.
Extrudates generally have long irregular cylindrical shapes, the length being restricted by naturally occurring fractures (for example extrusion presses). In addition to circular cross sections, star-shaped cross sections, for example, can also be produced in order to increase the external surface area. The extrudates have a lower density than tablets. Suitable sizes are diameters of about 1 mm or above. Hollow extrudates can also be produced.
Granules (grains) have a spherical shape. They are less dense than tablets and have a diameter of about 2 mm or more.
Tablets generally have a cylindrical shape, customary sizes being in the range from 5 mm to about 20 mm. Both cylinders and Raschig rings can be produced, with other shapes being possible to a limited extent.
Frequently, two compacting steps are required in shaping. The tablets are denser and stronger than extrudates or granules.
Suitable production processes and corresponding apparatuses are known for the individual catalyst shapes. In the abovementioned Catalyst Handbook it is stated that smaller particles are preferably produced by granulation or extrusion and not by tableting. The choice between extrusion and granulation accordingly principally depends on the type of the powder precursor.
For many catalyst applications, in particular in mobile systems, a high packing density of the catalyst in the catalyst bed is important, in order to ensure compact construction. In particular in the case of mobile systems for methanol reformation, in which methanol and steam produce hydrogen and carbon dioxide which are necessary for operating fuel cells, a high packing density or bulk density of the reformation catalyst is sought after. Catalysts which are suitable for this application are in particular particles made up of CuO, ZnO and Al2O3. Frequently, use is made of catalyst chips, which are obtained by comminuting catalyst tablets. By using catalyst chips, a high bulk density can be achieved, but the mechanical stability of the catalyst material is impaired, so that during operation, abrasion and pulverization and thus mechanical breakdown of the catalyst occur. In addition, there is a significantly higher pressure drop on beds of irregularly shaped chip particles than with beds of regularly shaped tablets.
EP-A-0 152 314 describes this catalyst chips of such a type having a particle diameter of less than 0.5 mm.
It is an object of the present invention to provide catalyst particles which permit a high bulk density and simultaneously have high mechanical stability. The catalysts are to be useable, in particular, for the steam reformation of methanol.